We are investigating amorphous bone mineral structure with the electron microscope using preparations that attempt to minimize artefacts. Freeze-fractured fresh bone, embedded freeze-dried bone powders and frozen chips of fresh bone are being examined, and the observations are being correlated with existing concepts of amorphous bone mineral structure. We are also investigating the response of bone cells to substances such as steroid hormones. The response is measured by the degree of incorporation by bone of H3-thymidine, H3-uridine, H3- proline, calcium 45 and sulfur 35. Autoradiographic procedures will be used to identify the cellular elements that are responsible for the isotope uptake. By these procedures we hope to be able to elucidate the sequence of events that occurs when bone progenitor cells and differentiated bone cells are stimulated to higher activity.